In general, in a case where a radome is placed over an antenna, in order to resolve the deterioration of antenna radiation characteristics due to the radome, a hemispherical radome curved surface is adopted so that even if the antenna is rotated, the incident angle of an electric wave becomes constant.
However, in an antenna with a radome mounted on a moving body such as an aircraft, since the height of the radome is restricted, the hemispherical curved surface of the constant incident angle as stated above can not be adopted, and accordingly, the loss due to the transmission of an electric wave through the radome, and the characteristic deterioration must be accepted.
That is, FIG. 1 is a schematic view showing an example of an antenna with a radome mounted on an aircraft. Since an antenna 1 and a radome 2 are mounted on the outside of an airframe 3, the anisotropic radome is adopted which has the lowest possible air resistance and has a streamline shape as shown in the drawing. Thus, when an electric wave is transmitted through the radome, as shown in FIG. 2, attenuation occurs in the output of the electric wave radiated from the antenna, and a shift occurs in the plane of polarization. For example, with respect to the electric wave having the plane of polarization of a horizontal polarization H1 and a vertical polarization V1 before the transmission through the radome, the shift of the plane of polarization by a certain phase angle as indicated by H2 and V2 can occur by the transmission through the radome, or the attenuation of the output as indicated by V2 can occur by the transmission through the radome. The degree of the shift of the plane of polarization and the attenuation of the output is largely affected by the frequency and directivity of the electric wave at that time, not to mention the position and shape of the radome.
FIG. 3 is a structural view of a conventional moving body satellite communication apparatus having a polarization plane control circuit disclosed in JP-A-2002-141849. In the drawing, a radome 2 is the foregoing anisotropic radome, and an antenna apparatus 1 includes a well-known main reflecting mirror 4, a secondary reflecting mirror 5, and a horn antenna 6. Reference numerals 7 and 8 denote 90° phase combiners for performing division into/composition of two channels while a phase of 90° is kept; 9a, 9b, 10a and 10b, variable phase shifters each inserted in a control system divided into the two channels and for phase shifting output signals of the 90° phase combiners 7 and 8; 11a and 11b, high-power amplifiers (HPA) for amplifying output signals of the variable phase shifters 9a and 9b; 12a and 12b, low-noise amplifiers (LNA) for amplifying output signals of an after-mentioned 90° phase combiner 14; 13, a 90° phase combiner for phase combining output signals of the high-power amplifiers 11a and 11b; 14, a 90° phase combiner for phase combining selection signals from after-mentioned diplexers 15 and 16; 15 and 16, diplexers for switching between transmission and reception and for separating/combining signals; 17, an orthomode transducer for functioning as an interface between the signal circuit and the antenna, which is also called a positive mode transducer; and 18, an antenna control circuit for performing a polarization angle adjustment of the antenna and other satellite tracking control of the antenna.
Next, the operation of this circuit will be described. In the case where the moving body satellite communication apparatus is mounted on an aircraft, since the relative positional relation to a satellite changes from moment to moment, it is necessary that the direction of a beam is always pointed toward the satellite by adjusting the polarization angle of the antenna 1. Now, when a transmitter signal to be transmitted to the satellite is inputted to a transmission side Tx terminal of FIG. 3, it is divided by the 90° phase combiner 7 into two channels having components orthogonal to each other, and the respective phases are independently controlled by the variable phase shifters 9a and 9b. The antenna control circuit 18 calculates the pointing direction of the antenna on the basis of the absolute position information of the aircraft and the position information of the satellite, and adjusts the phase quantities of the variable phase shifters 9a, 9b, 10a and 10b to achieve a desirable antenna polarization angle. Incidentally, the output electric wave from the antenna, that is, the effective isotropic radiated power (hereinafter referred to as EIRP) is kept at a definite value determined by a setting instruction value.
In the conventional antenna system as stated above, consideration is not given to an influence on the attenuation of an electric wave and the shift of the plane of polarization when it is transmitted through the anisotropic radome, and accordingly, no measures against this have been taken.